1. Field of the Invention
The present invention relates to an apparatus and method for controlling an inverter, and particularly, to an apparatus and method for controlling an inverter capable of enhancing reliability of measurement of a phase current even in a case where a time for which effective voltage vectors are applied is within a low modulation region as well as within a regular region.
2. Background of the Invention
Development of power semiconductor technologies and control technologies has allowed frequencies and voltages to be consecutively varied. Accordingly, a pulse width modulation (PWM) technique is greatly being used for adjusting speed in an alternating current (AC) system.
Specifically, attention is being given to a method for controlling an inverter adapting a PWM technique which modulates a space voltage vector. The PWM technique for modulating the space voltage vector can greatly control a high frequency current element in a normal state as compared to a commonly-used PWM technique for comparing triangular waves.
In the PWM technique for modulating the space voltage vector, gating signals can appropriately be generated according to sectors only after a reversely-converted actual switching time is calculated based on a time for which effective voltage vectors are applied according to the location of a reference voltage vector and on the location of the reference voltage vector.
In an inverter system using a single current sensor, restoring a phase current from a DC-link current sensor may be available only when there is provided a time enough to sample two effective vectors.
That is, one of the two effective voltage vectors is used to restore one phase current, and the other effective voltage vector is used to construct another phase current. As such, there may occur a problem in that the effective voltage vectors must be applied for a minimum time in order to obtain a reliable phase current.
In general, the minimum time for which the effective voltage vectors should be applied can be calculated by using Equation (1) as follows.Tmin=Tdead+Tsett+TAD  (1)
Here, Tdead denotes a dead time, Tsett denotes a stabilization time of a DC link ringing current generated due to switching, and TAD denotes an A/D conversion time.
FIG. 1 illustrates a method for measuring a phase current in case where effective vectors are located in a low modulation region according to the related art.
As illustrated in FIG. 1, the related art adapts a method in which a time for which the effective voltage vectors are applied is forcibly extended when values of T1/2 and T2/2 each obtained by use of a space voltage vector are smaller than a minimum time Tmin for restoring current.
In particular, a low-modulation region in which T1/2 and T2/2 are smaller than the minimum time Tmin at a time of starting (activating) an inverter system or during its low speed driving.
Accordingly, the related art, as aforementioned, uses the method for forcibly extending the applying time in order to measure the current.
However, in the related art, if the time for which the space voltage vector is applied is forcibly extended, errors of the space voltage vector may be generated. The errors of the space voltage vector may then occur distortion of the space voltage vector, which results in degradation of reliability of the normal low speed driving or reliability of the activation.